Rates of Chemical Reactions Vanderbilt Student Volunteers for Science Spring 2019

Why is the Science in this Lessons Important? Knowing and controlling the rate of reactions is important in living cells and industry. In the body chemical reactions must take place at the correct rate to supply your cells with exactly what they need when they need it. In industry, the products of chemical reactions make money so it is important to be able to speed up the rate and make them as cheaply as possible

I. Introduction Learning Goals: Students understand what is meant by “the rate of a reaction.” In Parts II-VII, students will understand the different factors that affect the rate of a reaction and why. In addition, they are able to discuss the opposite side of each factor, i.e. how to slow the reaction down. Set up: While one volunteer goes through the Introduction, the other volunteers need to fill 10 3.5 oz measuring cups (1 per group of 3) to the 50 mL line with ice. Ask class: “What is a chemical change?” Explain: “A chemical change does change the properties of a substance. One or more new substances are formed in a chemical change.” Ask class: “How can you tell when a chemical change has occurred?” Explain: “A color change, an emission of gas, or the formation of a precipitate. A precipitate is solid that comes from the mixture of two liquids. It can make the liquid look cloudy, or it could sit on the bottom.” The rate of a chemical reaction means how fast the reaction occurs. Factors affecting the rate include: temperature, concentration, surface area, and catalysts. Students may not know what a catalyst is: a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. See manual for further discussion questions and explanations

II. Effect of Temperature Learning Goals: Students understand the effect of temperature on the rate of a reaction. Hand out to each group of 3: Two tall clear cups Two small translucent cups – 1 containing ice and the other empty One packet of two effervescent tablets One plate Have the students fill both small cups to the 50 mL line with water and place them on the plate. Tell students to cut open one end of the packet of effervescent tablets. Have students add a whole tablet to each of the clear DRY cups. Make sure students realize the importance of adding the water at the same time. Then one of the VSVS team says "1,2,3, Go” and the students add all the water and ice water from their cups to the tall clear cups containing the tablets. See manual for further discussion questions

III. Effect of Surface Area: Dust in a Flame Learning Goals: Students understand surface area as a concept and can distinguish between larger (crushed tablets) and smaller (whole tablets) surface area reagents. Ask “What is surface area?” Surface area is the exposed surface of an object. See manual for sugar cube vs. crumbled sugar cube demonstration Demonstration: Show the students the lycopodium "dust” powder. Place a small pile of powder on the aluminum pan and attempt to light it with a match. (Depending on how long the match is held to the powder - it will either not burn or will burn enough to char a little.) Explain: “There is very little surface area for the dust to be reacted with, and so, very little reaction takes place.” Light the tea candle and place in aluminum pan. Load the pipette with a small amount of dust powder (enough to fill the tip). DO NOT turn the pipette upside down. Hold the pipette so the tip is near the flame and squeeze the pipette bulb to release the lycopodium powder into the flame. There will be a flash of fire. See manual for explanation.

III. Effect of Surface Area Ask students to use what they learned about surface area in the last experiment to suggest ways to increase surface of the tablets to speed up the rate of the reaction Experiment: Hand out: Two tall clear 10 oz cups (dry and unused) A ziploc bag Tablet packet Scissors Note: Students should already have two 3.5 oz. cups marked for 50 mL from first experiment Students: Fill the two cups small cups to the 50 mL mark Remove two effervescent tablets from the packet. Place one whole tablet in the bottom of one of the tall clear dry 10 oz plastic cups. Place one tablet into the ziploc bag and crush the tablet. Shake all the crushed tablet into one bottom corner of the bag and cut the other bottom corner off. Then, pour the crushed tablet through the cut corner into the other tall clear dry 10 oz plastic cup. *Continued on next slide

III. Effect of Surface Area: Particle Size Ask: Which of the two tablets has more surface area? (The crushed tablet - more of the inside surface of the tablet is now exposed). Which reaction do you predict will occur faster? Make sure students are ready to pour in the water from the smaller cups. Count them down 1,2,3, Go and have them add all the water to the tablets. Ask them which reaction they observed to happen faster.

IV. Effect of Concentration Learning Goals: Students understand the concept of concentration and how to tell how concentrated a liquid is. Demonstration of what Concentration means: Explain that concentration is the measure of how much of one substance is dissolved in another. Add 1 scoop of Koolaid powder to one of the cylinders Add 4 scoops to the other cylinder. Add water to both graduated cylinders and fill to the 100 mL mark. Hold the graduated cylinders up so students can see the difference in darkness of the purple color. Tell students that the 5% (weak) and 20% (strong) vinegar solutions (which will be used in the following demonstration) were prepared in a similar way.

IV. Effect of Concentration Student Activity: Hand out to each group: Two cups, A bottle of 20% vinegar A bottle of 5% vinegar Baking soda Refer to the vinegar as strong (20%) and weak (5%). Place the cups next to the solutions, and make sure the students are clear which strength they are using. Tell students to place a small spoonful of baking soda in each cup. There should be about the same amount of baking soda in each cup. Tell students they should be ready to pour the entire contents (vinegar) into the cups on the count of “1,2,3, Go.” Ask: “In which cup did the reaction occur with more bubbles?” Explain: “This reaction occurs quickly and with more bubbles in the strong (20%) solution and slower with fewer bubbles in the weak (5%) solution. This shows that the 20% (the solution of higher concentration) had more substance to react and therefore reacted faster.

Kit Clean-Up and Return It is important that all items be returned to the kit box. Be sure to collect all instruction sheets (in sheet protectors) and put them back in the kit box. Be careful not to place wet objects in kit. Kits should be returned to SC 5234 as soon as you return to campus from the school.